Graphene is a single layer sheet shaped structure that is composed of carbon atoms, and is a two dimensional material with one carbon-atom-thickness. Graphene shows excellent properties in many aspects. As an example, graphene is almost completely transparent and has a good light transmittance, by which only 2.3% of light is absorbed. A thermal conductivity of graphene reaches 5300 W/m·K, which is greater than carbon nanotube and diamond. An electron mobility of graphene at room temperature is greater than 15000 cm2/V·s, which is greater than carbon nanotube and silicon crystal. Graphene has the smallest electrical resistance in all known materials. An electrical resistivity of graphene is only about 10−6 Ω·cm which is lower than electrical resistivity of copper or silver. Besides, graphene is the most thin but hard material. Forming graphene into powder form is beneficial to the applications of graphene. For example, the graphene material powder can be used as an additive to improve electrical and mechanical properties of a high molecular material. The graphene material powder has broad application prospects.
Currently, graphene material powder can be produced by various methods, such as mechanical exfoliating method, oxidizing-reducing method, epitaxial growth method of crystal, chemical vapor deposition method, organic synthesizing method, and carbon nanotube exfoliating method. In these methods, the oxidizing-reducing method, which has a low cost and is easy to be processed, is the best way to form the graphene powder. However, the graphene powder formed by this method has a small density, which could be dusty in use, and is difficult to be stored and transported. Therefore, how to produce a graphene powder material that is beneficial to the applications in the downstream products is a crucial problem in the industrialization of graphene.
Chinese patent application No. 201310200469.5 discloses a method for making graphene material powder. In the method, graphite is added to and stirred uniformly in a mixing solution containing oxidizing and intercalating agents. The achieved mixing solution is ultrasonically processed while a helium gas is introduced to form a graphite intercalating compound that is intercalated with the intercalating agent and the helium moleculars. The graphite intercalating compound is filtered, washed, dried, and then heated in air. The heated graphite intercalating compound is dispersed in an organic solvent, and is ultrasonically exfoliated again while the helium gas is continuously introduced. The twice ultrasonically exfoliated graphite intercalating compound is then centrifugalized, deposited, filtered, washed, and then dried to achieve the graphene material powder. However, the graphene material powder formed by this method has a high aggregating degree and a poor dispersing ability, which are disadvantageous for the applications of the graphene material powder in the downstream products.
Chinese patent application No. 201010593157.1 discloses another method for making graphene material powder. In this method, an oxidized graphite is uniformly exfoliated to form a graphene oxide suspended liquid solution. The obtained graphene oxide suspended liquid solution is then sprayed to remove the solvent therein to achieve the graphite oxide powder. The graphene oxide powder experiences a non-expansion thermal treatment in an inert atmosphere or a reducing atmosphere to achieve the graphene material powder, which has a lower aggregating degree and a better dispersing ability. However, the graphene material powder formed by this method has a small density and a large volume, which are inconvenient for the storage and transportation. Under an external pressure, the graphene material powder formed by this method is easy to be re-stacked and is difficult to be restored to the original form. The graphene material powder can be dusty in use, which is disadvantageous for the applications in the downstream products.